U.S. patent number 7,085,499 [Application Number 10/116,801] was granted by the patent office on 2006-08-01 for agile rf-lightwave waveform synthesis and an optical multi-tone amplitude modulator.
This patent grant is currently assigned to HRL Laboratories, LLC. Invention is credited to Keyvan Sayyah, Daniel Yap.
United States Patent |
7,085,499 |
Yap , et al. |
August 1, 2006 |
Agile RF-lightwave waveform synthesis and an optical multi-tone
amplitude modulator
Abstract
A waveform synthesizer comprising for synthesizing RF lightwave
waveforms in the optical domain. These waveforms are constructed by
generating their constituent Fourier frequency components or tones
and then adjusting the amplitudes of those frequency components or
tones. The apparatus includes: a RF-lightwave frequency-comb
generator; and a multi-tone, frequency selective amplitude
modulator coupled to the RF-lightwave frequency-comb generator for
generating a continuous-wave comb comprising a set of RF tones
amplitude modulated onto a lightwave carrier.
Inventors: |
Yap; Daniel (Thousand Oaks,
CA), Sayyah; Keyvan (Santa Monica, CA) |
Assignee: |
HRL Laboratories, LLC (Malibu,
CA)
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Family
ID: |
26814635 |
Appl.
No.: |
10/116,801 |
Filed: |
April 5, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030090767 A1 |
May 15, 2003 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60332367 |
Nov 15, 2001 |
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Current U.S.
Class: |
398/183; 398/161;
398/186 |
Current CPC
Class: |
G02B
6/2861 (20130101); G02F 1/00 (20130101); H04B
10/2575 (20130101); H04B 10/505 (20130101); H04B
10/5051 (20130101); H04B 10/50577 (20130101); G02F
2203/56 (20130101) |
Current International
Class: |
H04B
10/04 (20060101); H04B 10/00 (20060101); H04B
10/12 (20060101) |
Field of
Search: |
;398/182,183,185,198,186,161,195 ;372/26,25,31,29.01,29.04 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 352 747 |
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Jan 1990 |
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EP |
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07-26136 |
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Jan 1995 |
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JP |
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07-264136 |
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Oct 1995 |
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JP |
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99/66613 |
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Dec 1999 |
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WO |
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00/44074 |
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Jul 2000 |
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WO |
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00/45213 |
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Aug 2000 |
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WO |
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00/45213 |
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Aug 2000 |
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WO |
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01/29992 |
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Apr 2001 |
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WO |
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01/80507 |
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Oct 2001 |
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WO |
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02/099939 |
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Dec 2002 |
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WO |
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03/0042734 |
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May 2003 |
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WO |
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03/043126 |
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May 2003 |
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WO |
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03/043177 |
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May 2003 |
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WO |
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03/043178 |
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May 2003 |
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WO |
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03/043195 |
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May 2003 |
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WO |
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03/043231 |
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May 2003 |
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WO |
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Primary Examiner: Chan; Jason
Assistant Examiner: Wang; Quan-Zhen
Attorney, Agent or Firm: Ladas & Parry LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. provisional application
No. 60/332,367 filed Nov. 15, 2001 for an "Agile RF-Lightwave
Waveform Synthesis and an Optical Multi-Tone Amplitude Modulator"
by Daniel Yap and Keyvan Sayyah, the disclosure of which is hereby
incorporated herein by reference.
This application is related to a provisional patent application
entitled "Agile Spread Waveform Generator" bearing Ser. No.
60/332,372 and filed Nov. 15, 2001, and its corresponding
non-provisional application bearing Ser. No. 10/116,829 and filed
on the same date as the present application, the disclosures of
which are hereby incorporated herein by this reference. These
related applications are owned by the assignee of this present
application.
This application is also related to a provisional patent
application entitled "Injection-seeding of a Multi-tone Photonic
Oscillator" bearing Ser. No. 60/332,371 and filed Nov. 15, 2001,
and its corresponding non-provisional application bearing Ser. No.
10/116,799 and filed on the same date as the present application,
the disclosures of which are hereby incorporated related
applications are owned by the assignee of this present
application.
This application is also related to a patent application entitled
"Remotely Locatable RF Power Amplification System" bearing Ser. No.
60/332,368 and filed Nov. 15, 2001, and its corresponding
non-provisional application bearing Ser. No. 10/116,854 and filed
on the same date as the present application, the disclosures of
which are hereby incorporated herein by this reference. These
related applications are owned by the assignee of this present
application.
This application is also related to a patent application entitled
"Waveguide-Bonded Optoelectronic Devices" bearing Ser. No.
60/332,370 and filed Nov. 15, 2001, and its corresponding
non-provisional application bearing Ser. No.10/116,800 and filed on
the same date as the present application, the disclosures of which
are hereby incorporated herein by this reference. These related
applications are owned by the assignee of this present application.
Claims
What is claimed is:
1. A waveform synthesizer comprising: (a) a RF-lightwave
frequency-comb generator for generating a continuous wave comb
comprising a set of RE tones modulated onto a lightwave carrier,
said set of RF tones comprising multiple upper and lower modulation
sideband pairs, the RF-lightwave frequency-comb generator
comprising multiple loops each having an optical delay line, the
optical delay lines in the different loops having different
lengths, at least one photodetector, and an optical intensity
modulator, the optical delay lines receiving an optical output of
the optical intensity modulator, the output of the optical
intensity modulator also being supplied to the multi-tone,
frequency selective modulator, the outputs of the optical delay
lines being detected by said at least one photodetector, the output
of the detector being coupled to the optical intensity modulator,
wherein spacing between RF tones in said set of RF tones is
determined by a delay through a shortest of the optical delay
lines; (b) a multi-tone, frequency selective modulator coupled to
the RF-lightwave frequency-comb generator and configured to produce
a modulated RF-lightwave frequency comb; and (c) a photodetector
for producing a synthesized RE waveform from said lightwave carrier
and said multiple upper and lower modulation sideband pairs in said
modulated RF-lightwave frequency comb.
2. The waveform synthesizer of claim 1 wherein the amplitudes of
the RF tones are given different weights by the frequency selective
modulator and wherein values of said weights are changeable.
3. The waveform synthesizer of claim 1 wherein the generator of the
RF-lightwave frequency comb comprises a photonic oscillator.
4. The waveform synthesizer of claim 1 wherein the multi-tone,
frequency selective modulator is coupled to receive the output of
the RF-lightwave frequency-comb generator by a waveguide and
wherein the frequency selective modulator includes a set of
frequency selective optical reflectors or couplers which interact
with said waveguide.
5. The waveform synthesizer of claim 4 wherein said set of
frequency selective optical reflectors or couplers which interact
with said waveguide include an outlet waveguide segment for
conducting light away from said waveguide.
6. The waveform synthesizer of claim 4 wherein said set of
frequency selective optical reflectors or couplers which interact
with said waveguide include a set of circular-shaped resonators
each having a different diameter and a corresponding different
resonant optical frequency which corresponds to a tone in said set
of RF tones.
7. The waveform synthesizer of claim 4 wherein said set of
frequency selective optical reflectors or couplers which interact
with said waveguide include a set of distributed-feedback
resonators each having a different resonant optical frequency
corresponding to a tone in said set of RF tones.
8. The waveform synthesizer of claim 4 wherein said set of
frequency selective optical reflectors or couplers which interact
with said waveguide include a set of circular-shaped resonators
each having a frequency control input for receiving a control
signal, the control signals delivered to said set of frequency
selective optical reflectors or couplers causing each of said
frequency selective optical reflectors or couplers to assume a
corresponding different resonant optical frequency which
corresponds to a tone in said set of RF tones.
9. The waveform synthesizer of claim 4 wherein said set of
frequency selective optical reflectors or couplers which interact
with said waveguide include a set of distributed-feedback
resonators each having a frequency control input for receiving a
control signal, the control signals delivered to said set of
distributed-feedback resonators causing each of said
distributed-feedback resonators to assume a corresponding different
resonant optical frequency which corresponds to a tone in said set
of RF tones.
10. The waveform synthesizer of claim 1 wherein the multi-tone,
frequency selective modulator is coupled to receive the output of
the RF-lightwave frequency-comb generator by an input waveguide and
further including an output waveguide coupled to the input
waveguide by associated pairs of resonators, one resonator in each
of said associated pairs of resonators coupling light from said
input waveguide and the other resonator in each of said associated
pairs of resonators coupling light into said output waveguide.
11. The waveform synthesizer of claim 10 wherein each associated
pair of resonators is coupled to a modulator for modulating the
amplitude and/or phase of light coupled from the input waveguide to
the output waveguide.
12. The waveform synthesizer of claim 1 wherein the at least one
photodetector comprises multiple photodetectors with one
photodetector in each loop.
13. The waveform synthesizer of claim 1 wherein the RF-lightwave
frequency-comb generator comprises multiple loops including: (i) a
first optical delay line in a first loop for spacing a comb
generated by the a multi-tone optical comb generator; (ii) a second
optical delay in a second loop line for noise reduction, the second
delay line being longer than the first optical delay line; (iii) at
least one photodetector connected to the first and second delay
lines; and (iv) an optical intensity modulator in a loop portion
common to the first and second loops for driving the first and
second optical delay lines.
14. The waveform synthesizer of claim 13 wherein the loop common
portion further includes an amplifier and a band pass filter.
15. The waveform synthesizer of claim 14 wherein the amplifier is
an electronic amplifier.
16. The waveform synthesizer of claim 13 wherein the loop common
portion further includes a band pass filter and wherein at least
one of the first and second loops includes an optical amplifier
therein.
17. The waveform synthesizer of claim 13 further including means
for compensating for environmental changes affecting a length of at
least one of the first and second optical delay lines.
18. The waveform synthesizer of claim 17 wherein the means for
compensating for environmental changes affecting the length of at
least one of the first and second optical delay lines comprises an
apparatus for adjusting the length of at least one of the first and
second optical delay lines and a feedback circuit including a tone
selection filter coupled to the loop common portion and a mixer for
mixing the output of the tone selection filter with a reference
signal, an output of the mixer being operatively coupled to the
length adjusting apparatus.
19. The waveform synthesizer of claim 18 wherein the length
adjusting apparatus also adjusts the length of the first and second
optical delay lines.
20. The waveform synthesizer of claim 18 wherein the optical
intensity modulator is an electroabsorption modulator having an
electrical output and the tone selection filter is coupled to the
electrical output of the electroabsorption modulator.
21. The waveform synthesizer of claim 17 wherein the means for
compensating for environmental changes affecting the length of at
least one of the first and second optical delay lines comprises a
phase shifter disposed in the loop common portion and a feedback
circuit including a tone selection filter coupled to the loop
common portion and a mixer for mixing an output of the tone
selection filter with a reference signal, an output of the mixer
being operatively coupled to the phase shifter.
22. The waveform synthesizer of claim 21 wherein the optical
intensity modulator is an electroabsorption modulator having an
electrical output and the tone selection filter is coupled to the
electrical output of the electroabsorption modulator.
23. The waveform synthesizer of claim 1 further comprising: a
second RF-lightwave frequency-comb generator for generating a
continuous wave comb comprising a second set of RF tones modulated
onto a second lightwave carrier, said second set of RF tones
comprising multiple upper and lower modulation sideband pairs; and
a second multi-tone, frequency selective modulator coupled to the
second RF-lightwave lightwave frequency-comb generator and
configured to produce a second modulated RE-lightwave lightwave
frequency comb.
24. The waveform synthesizer of claim 23 wherein the lightwave
carrier and the second lightwave carrier are of different
wavelength.
25. The waveform synthesizer of claim 23 further comprising: a
second photodetector for producing a synthesized RE waveform from
said second lightwave carrier and said multiple upper and lower
modulation sideband pairs in said second modulated RF-lightwave
frequency comb.
26. The waveform synthesizer of claim 25 further comprising:
differential amplifier adapted to combine outputs of the
photodetector and the second photodetector.
27. The waveform synthesizer of claim 25 further comprising: a
first optical modulator coupled to the multi-tone, frequency
selective modulator and adapted to modulate a RF signal onto the
modulated RE-lighwave frequency comb; a second optical modulator
coupled to the second multi-tone, frequency selective modulator and
adapted to modulate a phase inverted RE signal onto the second
modulated RF-lighwave frequency comb.
28. A method of synthesizing a frequency translated RF-modulated
multi-tone lightwave waveform in the lightwave domain as well as a
corresponding RF waveform in the RF domain, the method comprising:
generating a single frequency optical carrier; generating RF
modulation sideband tones on said optical carrier in the lightwave
domain; adjusting amplitudes of said sideband tones in the
lightwave domain; generating a single-tone lightwave reference; and
heterodyning said RF modulation sideband tones on said optical
carrier with said single-tone lightwave reference.
29. The method of claim 28 wherein generating RF modulation
sideband tones on said optical carrier comprises: generating a
RF-lightwave frequency-comb in multiple loops each having an
optical delay line, the optical delay lines in the different loops
each having an output and having different lengths, photodetecting
light at the outputs of the optical delay lines to thereby produce
photodetected signals, and applying the photodetected signals to an
optical intensity modulator, the at least two optical delay lines
receiving an optical output of the optical intensity modulator and
the output of optical intensity modulator also being supplied to
the multi-tone, frequency selective amplitude modulator.
30. The method of claim 29 wherein the at least one photodetector
comprises multiple photodetectors with a separate photodetector in
each loop.
31. The method of claim 28 wherein the amplitudes of the sideband
tones are adjusted using a set of frequency selective optical
reflectors or couplers which interact with the lightwave domain
waveform in a waveguide.
32. The method of claim 31 wherein said set of frequency selective
optical reflectors or couplers interact with said waveguide to
conduct light away from said waveguide.
33. The method of claim 31 wherein the waveguide is an input
waveguide and further including a output waveguide and wherein the
set of frequency selective optical reflectors or couplers are
arranged to interact with said input waveguide to conduct light
away from said input waveguide and into said output waveguide.
34. The method of claim 33 wherein the light coupled by the set of
frequency selective optical reflectors or couplers has its
amplitude and/or phase modulated by a set of modulators.
35. The method of claim 31 wherein said set of frequency selective
optical reflectors or couplers which interact with said waveguide
include a set of circular-shaped resonators each having a different
diameter and a corresponding different resonant optical frequency
which corresponds to a frequency in said component frequencies of
the waveform in the lightwave domain.
36. An optical multi-tone generator comprising: a first optical
delay line in a first loop for spacing a comb generated by the a
multi-tone optical comb generator; a second optical delay in a
second loop line for noise reduction, the second delay line being
longer than the first optical delay line; at least one
photodetector adapted to detect outputs of the first and second
delay lines; an optical intensity modulator in a loop portion
common to the first and second loops for driving the first and
second optical delay lines; and means for compensating for
environmental changes affecting a length of at least one of the
first and second optical delay lines, wherein the means for
compensating for environmental changes affecting the length of at
least one of the first and second optical delay lines comprises a
phase shifter disposed in the loop common portion and a feedback
circuit including a tone selection filter coupled to the loop
common portion and a mixer for mixing the output of the tone
selection filter with a reference signal, an output of the mixer
being operatively coupled to the phase shifter.
37. An optical multi-tone generator of claim 36 wherein the output
of optical intensity modulator is being supplied to a multi-tone,
frequency selective amplitude modulator.
38. The optical multi-tone generator of claim 36 wherein the at
least one photodetector comprises multiple photodetectors with one
photodetector in each loop.
39. The optical multi-tone generator of claim 36 wherein the loop
common portion further includes an amplifier and a band pass
filter.
40. The optical multi-tone generator of claim 39 wherein the
amplifier is an electronic amplifier.
41. The optical multi-tone generator of claim 36 wherein the loop
common portion further includes a band pass filter and wherein at
least one of the first and second loops includes an optical
amplifier therein.
42. The optical multi-tone generator of claim 36 wherein the
optical intensity modulator is an electroabsorption modulator
having an electrical output and the tone selection filter is
coupled to the electrical output of the electroabsorption
modulator.
43. An optical multi-tone generator comprising: a first optical
delay line in a first loop for spacing a comb generated by the a
multi-tone optical comb generator; a second optical delay in a
second loop line for noise reduction, the second delay line being
longer than the first optical delay line; at least one
photodetector adapted to detect outputs of the first and second
delay lines; an optical intensity modulator in a loop portion
common to the first and second loops for driving the first and
second optical delay lines; and means for compensating for
environmental changes affecting a length of at least one of the
first and second optical delay lines, wherein the means for
compensating for environmental changes affecting the length of at
least one of the first and second optical delay lines comprises an
apparatus for adjusting the length of at least one of the first and
second optical delay lines and a feedback circuit including a tone
selection filter coupled to the loop common portion and an
electronic mixer for mixing the output of the tone selection filter
with a reference signal, an output of the electronic mixer being
operatively coupled to the length adjusting apparatus.
44. The optical multi-tone generator of claim 43 wherein the
optical intensity modulator is an electroabsorption modulator
having an electrical output and the tone selection filter is
coupled to the electrical output of the electroabsorption
modulator.
45. The optical multi-tone generator of claim 43 wherein the length
adjusting apparatus adjusts the lengths of the first and second
optical delay lines.
46. A method of synthesizing a frequency translated RE-modulated
multi-tone lightwave waveform in the lightwave domain as well as a
corresponding RE waveform in the RE domain, the method comprising:
generating a continuous wave comb using an RE-lightwave
frequency-comb generator, the continuous wave comb comprising a set
of RE tones modulated onto a lightwave carrier, said set of RE
tones comprising multiple upper and lower modulation sideband
pairs, the RE-lightwave frequency-comb generator comprising
multiple loops each having an optical delay line, the optical delay
lines in the different loops having different lengths, at least one
photodetector, and an optical intensity modulator, the optical
delay lines receiving an optical output of the optical intensity
modulator, the output of the optical intensity modulator also being
supplied to the multi-tone frequency selective modulator, the
outputs of the optical delay lines being detected by said at least
one photodetector, the output of the detector being coupled to the
optical intensity modulator, wherein spacing between RE tones in
said set of RE tones is determined by a delay through a shortest of
the optical delay lines; generating a modulated RE-lightwave
frequency comb based on the continuous wave comb; and generating a
synthesized RF waveform from said lightwave carrier and said
multiple upper and lower modulation sideband pairs in said
modulated RF-lightwave frequency comb.
47. A method of claim 46, further comprising: generating a second
continuous wave comb comprising a second set of RF tones modulated
onto a second lightwave carrier, said second set of RE tones
comprising multiple upper and lower modulation sideband pairs;
generating a second modulated RF-lightwave frequency comb based on
the second continuous wave comb; and generating a second
synthesized RE waveform from said second lightwave carrier and said
multiple upper and lower modulation sideband pairs in said second
modulated RF-lightwave frequency comb.
Description
TECHNICAL FIELD
This invention relates to optical techniques for synthesizing
RF-modulated lightwave waveforms as well as corresponding RF
waveforms. By following the teachings of this invention, a variety
of wideband RF-modulated lightwave waveforms can be synthesized in
the form of amplitude modulation tones on a lightwave carrier.
These waveforms are constructed by generating the component
frequencies of the waveform in the lightwave domain and by
adjusting the amplitudes of those components. A RF waveform can
then be obtained by photodetection of the modulated lightwave
waveform. Furthermore, a frequency-shifted version of the RF
waveform can be obtained by using optical heterodyning techniques
that combine the modulated multi-tone lightwave waveform with a
single-tone lightwave reference.
Also disclosed are two embodiments of an optical multi-tone
amplitude modulator, which finds use in the disclosed optical
circuits and optical techniques.
BACKGROUND OF THE INVENTION
The present invention relates to an optical method of synthesizing
arbitrary RF-lightwave or RF waveforms. Prior methods can generate
multiple RF tones but have no provision for selectively adjusting
the amplitudes of those tones. Known methods can be used to
generate multi-tone RF combs amplitude-modulated on lightwave
carriers. This invention improves upon such known techniques by
filtering select lightwave frequencies and applying them to
amplitude modulate the tones of the comb.
Prior art digital electronic synthesizers are quite versatile, but
can produce waveforms that have bandwidths of only several hundred
megahertz. Analog electronic synthesizers are capable of higher
bandwidths, as high as several tens of gigahertz, but the waveforms
are quite simple (comprised of only a few tones). The disclosed
optical methods and apparatus of this invention, which allow for
synthesizing the waveforms while in the lightwave domain, can
produce waveforms with bandwidths in excess of one terahertz and
that are comprised of a large number of tones.
The prior art includes the following:
(1). 1.8-THz bandwidth, tunable RF-comb generator with
optical-wavelength reference--see the article by S. Bennett et al.
Photonics Technol. Letters, Vol. 11, No. 5,pp. 551 553, 1999. This
paper describes multi-tone RF-lightwave comb generation using the
concept of successive phase modulation of a laser optical waveform
in an amplified circulating fiber loop. A phase modulator in an
amplified re-circulating fiber loop generates the RF-lightwave
frequency comb. In this comb generator, the lightwave signal from a
laser injected into an optical loop undergoes phase modulation and
optical amplification on each round trip. A series of optical
sidebands spaced exactly by the RF modulation frequency applied to
the phase modulator are generated.
(2). Multi-tone operation of a single-loop optoelectronic
oscillator--see an article by S. Yao and L. Maleki, IEEE J. Quantum
Electronics, v.32,n.7,pp.1141 1149, 1996. This document discloses a
single loop optoelectronic oscillator. This oscillator contains a
modulator, optical feedback loop, and photodetector. Although the
intent of the authors is to generate a single tone by incorporating
a narrow-band frequency filter in the loop, demonstration of
multiple tones was achieved by enlarging the bandwidth of the
filter. The frequency spacing of those multiple tones was set by
injecting a sinusoidal electrical signal into the modulator, with
the frequency of the injected signal equal to the spacing of the
tones. This method causes all of the oscillator modes (one tone per
mode) to oscillate in phase. (3). Micro-ring resonators with
absorption tuning for wavelength selective lightwave add/drop
filtering--see the articles by S. T. Chu, B. E. Little, et al.,
IEEE Photonics Technol. Letters, Vol. 11,No. 6,pp. 691 693, 1999
and by B. E. Little, H. A. Haus, et al., IEEE Photonics Technol.
Letters, Vol. 10,No. 6,pp. 816 818, 1998. The Chu article provides
experimental results verifying that a collection of micro-ring
resonators can be used to separately filter a series of lightwave
frequencies (or wavelengths). The second article provides an
analysis that indicates the absorption, or loss, of the micro-ring
resonator can be used to change the amount of light that is coupled
into a micro-ring resonator and, thus, filtered.
(4). Optical add/drop filters based on distributed feedback
resonators--see the papers by R. F. Kazarinov, C. H. Henry and N.
A. Olsson, IEEE J. Quantum Electron. Vol. QE-23,No. 9,pp. 1419
1425, 1987 and by H. A. Haus and Y. Lai, J. Lightwave Technol.,
Vol. 10,No. 1,pp. 57 61, 1992. This paper provides the design for
another type of optical filter that can have RF bandwidths. The
design provides for a filter bandwidth of 10 GHz. Even smaller
bandwidths could be realized using currently available fabrication
techniques. The authors do not discuss how to change the amount of
light that is filtered.
(5). Optical add/drop filters based on Bragg gratings in
interferometers--see the paper by F. Bilodeau, et al., IEEE
Photonics Technol. Letters, Vol. 7,No. 4,pp. 388 390, 1995. This
paper describes the use of Bragg gratings in an optical-fiber
interferometers configuration to accomplish the add/drop filtering.
The authors do not discuss how to change the amount of light that
is filtered. The filtering bandwidth of an optical Bragg grating is
quite broad. A FWHM bandwidth of 25 GHz was reported for a Bragg
grating of 1-cm length.
The waveform synthesizer disclosed herein includes a RF-lightwave
frequency-comb generator that is coupled to a multi-tone, frequency
selective amplitude modulator. The continuous-wave (CW) comb is a
set of RF tones that are amplitude modulated onto a lightwave
carrier. The amplitudes of these RF tones can be given different
weights by the frequency-selective modulator, and the values of
these weights can be changed. Since a waveform is described by its
Fourier spectrum, which is the amplitudes of its constituent
frequency components, changing the values of these amplitudes will
change the waveform that results. The generator of the RF-lightwave
frequency comb is preferably a photonic oscillator or,
alternatively, a single loop optoelectronic oscillator or a tunable
re-circulating comb generator, the latter two of which are known
per se in the art. The amplitude weights are applied preferably by
a set of wavelength or frequency selective optical reflectors or
couplers.
The present invention makes use of a single-tone RF reference to
synthesize a variety of wideband RF-lightwave and RF waveforms. The
RF lightwave waveform can be carried on optical fiber or
transmitted through free-space optical links. The RF waveform is
constructed by demodulating the complete RF waveform from a
lightwave carrier using a photodetector. The highest frequency
component of the synthesized waveform can have a frequency that is
substantially higher than that of the RF reference.
Agile wideband waveforms are especially useful for optical
communication systems with multiple users and for secure optical
links. For example, each user can be assigned a particular and
unique pattern for the amplitudes of the tones in the waveform. A
user can then distinguish its signal from other signals that occupy
the same band of frequencies by coherently processing the received
signal with a copy of the particular waveform pattern of that user.
This type of Code Division Multiple Access (CDMA) for lightwave
waveforms is different from prior art techniques. Prior techniques
make use of short optical pulses, much shorter than the information
pulse, whose wavelength and sequence of temporal locations can be
different for each user. The waveforms synthesized by the approach
of this invention also could be used for wideband RF communications
systems and links.
Agile, wideband waveforms can serve as carrier waveforms for
low-probability of intercept (LPI) radar systems. The capability
for amplitude weighing of the individual tones of the frequency
spread multi-tone waveform provides a significant enhancement over
the invention described in the patent application entitled "Agile
Spread Waveform Generator" which is discussed above.
BRIEF DESCRIPTION OF THE INVENTION
In one aspect, the present invention provides a waveform
synthesizer comprising: a RF-lightwave frequency-comb generator;
and a multi-tone, frequency selective amplitude modulator coupled
to the RF-lightwave frequency-comb generator for generating a
continuous-wave comb comprising a set of RF tones amplitude
modulated onto a lightwave carrier.
In another aspect, the present invention provides a method of
synthesizing a RF-modulated lightwave waveform as well as a
corresponding RF waveform, the method comprising the steps of:
generating component frequencies of the waveform in the lightwave
domain; adjusting the amplitudes of those components in the
lightwave domain; and generating a RF waveform by photodetecting
the modulated lightwave waveform.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating one embodiment of an agile
waveform synthesizer, which can produce both RF-lightwave and RF
waveforms;
FIG. 2 is a block diagram which bears some resemblance to FIG. 1
with the synthesizer thereof being modified, in this embodiment, to
accommodate both positive and negative coefficients;
FIG. 3 is a block diagram illustrating a second embodiment of an
agile waveform synthesizer, which can produce a heterodyne
RF-lightwave waveform, and a frequency translated RF waveform;
FIG. 4 is a block diagram illustrating one embodiment of a tunable,
multi-tone, optical comb generator;
FIG. 5 is a block diagram illustrating a second embodiment of a
tunable, multi-tone, optical comb generator;
FIGS. 6(a) and 6(b) illustrates two embodiments of a multi-tone
amplitude modulator which are based on circular microresonators
having electrically controlled optical absorption; and
FIGS. 7(a) and 7(b) illustrates two additional embodiments of a
multi-tone amplitude modulator, these embodiments being based on
distributed-feedback optical resonators with electrically
controlled optical leakage or pass through.
DETAILED DESCRIPTION
This invention presents an approach for synthesizing RF lightwave
waveforms in the optical domain. These waveforms are constructed by
generating their constituent Fourier frequency components or tones
and then adjusting the amplitudes of those frequency components or
tones. A RF waveform can be produced from the RF-lightwave waveform
by photodetection. Furthermore, a frequency-shifted RF waveform can
be produced by heterodyne detection for which a second, single-tone
lightwave signal with the proper wavelength offset also is supplied
to a photodetector. See also the related U.S. patent provisional
application Ser. No. 60/332,372 and filed Nov. 15, 2001, and its
corresponding non-provisional application bearing Ser. No.
10/116,829 filed on the same date as the present 236 application,
both of which are entitled "Agile Spread Waveform Generator" and
both of which are mentioned above.
A time-varying waveform can be described in terms of its Fourier
spectrum. This spectrum consists of various frequency tones with
each tone having some specific amplitude. Typically, a waveform is
generated by some means and it is characterized by examining its
Fourier spectrum with an electronic or optical spectrum analyzer.
However, if there is some way to generate the various frequency
tones that comprise a waveform and to give those tones the desired
amplitudes, the waveform itself can be synthesized. This invention
provides a way to do such synthesis of waveforms in the optical
domain.
One embodiment of the waveform synthesizer of this invention is
illustrated by the block diagram of FIG. 1. This synthesizer
includes a lightwave source, such as laser 10, an optical comb
generator 14, and a multi-tone amplitude modulator 15. Light from
the laser 10 is preferably in the form of a single tone (single
wavelength), which can be considered as a lightwave carrier. This
carrier is supplied to the optical comb generator or oscillator 14.
The comb generator/oscillator 14 produces a set of additional tones
that are amplitude-modulated onto the lightwave carrier. These
tones have some fixed amplitude distribution that is determined by
the comb generator 14. The multi-tone lightwave waveform is then
supplied to the multi-tone amplitude modulator 15. This modulator
can adjust the amplitude of each tone individually, to match the
amplitude profile of the desired waveform. The output of the
amplitude modulator 15 is the RF-lightwave waveform. This
RF-lightwave waveform can serve as an encoded carrier onto which a
RF signal is modulated by an optical modulator 13. The RF signal
can be a polyphase code or a pulse code, for example. The
RF-lightwave waveform also can be directed to a photodetector 18.
The output of the photodetector 18 is a RF version of the
synthesized waveform, with the original lightwave carrier (supplied
by the laser) removed and the two amplitude-modulation sidebands
combined. An electronic reference oscillator 24 can be optionally
included in the synthesizer to help control the frequency spacing
of the comb. In addition, an amplitude controller circuit 17 can be
included to adjust, in real time, the amplitudes of the individual
tones, thereby permitting the RF-lightwave waveform to change its
shape.
The encoded carrier generated by the apparatus of FIG. 1 may be
utilized, for example, in connection with CDMA based communication
systems. Each user or community channel of the CDMA based
communication system is assigned a particular carrier comprised of
multiple tones of specific amplitudes. By using the disclosed
synthesizer to produce unique combinations of amplitudes for those
tones, each carrier can be made unique. In a communication system,
a waveform synthesizer is located at or near the transmitted unit
and a complimentary waveform synthesizer is located with the
receiving unit. Both synthesizers are set to produce the same
amplitude modulated pattern for the tones. In general, the
frequency spacing of the multiple tones comprising the synthesized
waveform is larger than the maximum frequency content of the RF
signal modulated onto modulator 13. The encoded carrier generated
by the embodiments yet to be discussed can also be utilized for the
same purposes.
The waveform synthesizer of FIG. 1 adjusts the amplitude of each
tone according to coefficients so long as the coefficients have a
common sign (plus or minus). FIG. 2 is a more versatile embodiment
of the waveform synthesizer that can accommodate both positive and
negative coefficient values. In FIG. 2 the laser 10, comb generator
14, the reference oscillator 24, the amplitude controller 17, the
multi-tone amplitude modulator 15 and the modulator 13 appear twice
with either a -1 or a -2 being added to the element numerals. The
-1 elements are associated with laser 10-1 which outputs laser
light at a frequency .lamda..sub.1 while the -2 elements are
associated with laser 10-2 which outputs laser light at a frequency
.lamda..sub.2. The modulators 13-1 and 13-2 both receive the RF
signal, but in comparatively phase inverted forms. The phase
inversion is accomplished, in this embodiment, by utilizing a phase
invertor 13A in the signal path to one of the two modulators,
modulator 13-2. The modulated outputs of the modulators 13-1 and
13-2 are combined by a wavelength division multiplexing (WDM)
combiner 13B. The output of the combiner 13B provides a lightwave
version of output waveform. In order to provide an electrical
version thereof, a WDM splitter 18A splits the lightwave signal
back up into its .lamda..sub.1 and .lamda..sub.2 components that
are separately photodetected by detectors 18B and 18C,
respectively. The outputs of detectors 18B and 18C are combined by
a differential adder/amplifier 18D. The output of element 18D is
the electrical version of the RF waveform output.
Another variation of the waveform synthesizer of FIG. 1 is
illustrated by FIG. 3. This variation synthesizes a
dual-line-carrier RF-lightwave waveform and a frequency-translated
RF waveform. A second lightwave source, such as another single-tone
laser 11, is added to the synthesizer of FIG. 1. This second laser
11 produces a reference lightwave carrier whose wavelength is
offset from the wavelength of the first laser 10. The wavelength
difference (or offset) can be considered as a local-oscillator
frequency. Both the comb-modulated carrier (of the first laser 10)
and the reference carrier (of the second laser 11) impinge upon a
square-law photodetector 18. The photodetector 18 produces an
electrical signal that is the heterodyne combination of both
incident RF-lightwave signals. This electrical signal is translated
in frequency by an amount equal to the wavelength offset of the two
lasers 10, 11. The process of heterodyning, outlined above, is well
known. In order for the heterodyne to be stable, the two lasers
must be phase locked. A phase locking module 13 is preferably
provided to perform this function. Various methods known in the art
can be used to achieve the phase locking. These methods include
optical injection locking of the two lasers (the slave lasers) to:
(1) different modes of a mode-locked master laser, (2) modulation
sidebands of a frequency or phase modulated master laser, or (3)
different phase-locked modes of an optical comb generator. Another
method makes use of a phase-lock loop that takes the heterodyne
output of the two lasers, before they are modulated by the comb,
and compares that output with an external RF reference in a RF
phase detector to produce an error signal for correcting the
wavelengths of the lasers. In all these methods, a highly stable
and low phase-noise RF reference oscillator is used to externally
lock the mode locked laser or the optical comb generator, to
provide the modulation sidebands in the phase modulated laser, or
to provide a reference for the phase-lock loop. Both the
heterodyning process and the methods for producing phase-locked
lasers are discussed in more detail in the patent application
entitled "Agile Spread Waveform Generator" discussed above.
The embodiment of FIG. 3 can be further modified to accommodate
both positive and negative coefficient values. This would be done
by making the same sort of modification done in the embodiment of
FIG. 2 relative to FIG. 1 to FIG. 3 instead. This would involve
providing -1 and -2 version of elements 10, 11, 13, 14, 15, 17, 24
and 26 (corresponding to frequencies .lamda..sub.1 and
.lamda..sub.2) and with the outputs of the -1 and -2 versions of
the modulators 13 being applied to the input of the WDM combiner
13B as previously discussed with respect to FIG. 2.
The multi-tone comb generator 14 can be implemented using a variety
of known devices. Two of these devices are a single loop,
electrically injection-locked optoelectronic oscillator and a
re-circulating optical-comb generator, both of which devices are
known per se in the prior art. The preferred comb generator 14 is a
multi-loop, multi-tone photonic oscillator, one embodiment of which
is depicted by FIG. 4. A photonic oscillator is expected to produce
tones that have lower phase noise than the re-circulating comb
generator. Moreover, the tones produced by this photonic oscillator
need not be mutually coherent and, thus, those tones can be
combined at the photodetector 18, as is shown in FIG. 3, without
the generation of additional beat tones. However, in some cases, it
may be desirable to generate tones that are phase locked to each
other. A series of pulses is thereby produced rather than a
continuous-wave (CW) waveform. In those cases a re-circulating comb
generator or the single loop, electrical injection-locked
optoelectronic oscillator mentioned in the prior art documents
noted above could be used as the multi-tone comb generator 14.
An improved version of a multi-loop, multi-tone photonic comb
generator or oscillator is illustrated by FIG. 4. This embodiment
of a photonic comb generator or oscillator 14 includes an optical
intensity modulator 32, lightwave delay paths 34 and 36, two
photodetectors 38 and 40, a low-noise electrical amplifier 42, an
electrical phase shifter 85 and a RF bandpass filter 44. Light from
the laser 10, which supplies power for this oscillator, is
modulated by a RF signal at an electrical input 32a of the
modulator 32. The modulated lightwave is then split into two
branches, one connected to a shorter optical delay path 34, and the
other to a longer optical delay path 36. The RF-lightwave signals
in the two optical paths 34 and 36 are photodetected by
photodetectors 30 and 40 and then combined. The electrical outputs
of the photodetectors are combined and preferably amplified by an
amplifier 42 and bandpass filtered by a filter 44 and then fed back
to the modulator, as shown in FIG. 4. The bandpass filter 44 sets
the RF bandwidth of the generated RF multi-tone comb spectrum.
Random noise generated in the feedback loops modulates the laser
light, which after propagating through the two optical delay paths
and being photodetected is regeneratively fed back to the
modulator. Potential oscillation modes exist at frequency intervals
that are an integer multiple of the inverse of the delay times of
the two loops (.tau..sub.S and .tau..sub.L), where .tau..sub.S is
the delay time of the shorter loop and .tau..sub.L is the longer
loop's delay time. However, oscillation will only occur at
frequencies where the modes resulting from both delay loops overlap
if the sum of the open loop gains of both feedback loops is greater
than one and the open loop gains of each feedback loop is less than
one. Therefore, oscillation will only occur at modes spaced at the
frequency interval determined by the shorter loop
(.DELTA.f=k/.tau..sub.S). On the other hand, the oscillator phase
noise S(f') decreases quadratically with the optical delay time in
the longer loop: S(f')=.rho./[(2.pi.).sup.2(.tau..sub.Lf').sup.2],
where .rho. is the input noise-to-signal ratio and f' is the offset
frequency. Combining these two effects results in a multi-tone,
multi-loop photonic oscillator in which the tone spacing and phase
noise can be controlled independently. Additional discussion of
this photonic oscillator is provided in the provisional and
non-provisional patent applications entitled "Agile Spread Waveform
Generator" referenced above. Instead of using an electronic
amplifier 42 in the electronic portion of the loops, an optical
amplifier 401 or 402 can be used in the optical portion of the
loops.
The device shown in FIG. 4 contains a means for tuning the
frequencies of the RF tones. As shown in FIG. 4, an electronic
phase shifter 85 is added in the feedback loop of this embodiment
of the generator/oscillator 14 for tuning the frequencies of the RF
tones generated thereby. A frequency divider 84 and a tone-select
filter 83 selects one tone of the comb to compare with a tunable
external reference 24. An electronic mixer 86 acts as a phase
detector the output of which is filtered by a filter 88 to produce
an error signal 90 used to control the phase shifter 85. Note that
changing the phase of the phase shifter 85 changes the phase delay
incurred by propagation through both the shorter and longer
feedback loops of the oscillator. This type of phase-lock loop
control is known per se in the art. The precise frequency and
frequency spacing of the oscillator tones is changed, therefore, by
changing the frequency of the external reference 24. The rate with
which the external reference frequency is tuned should be slower
than the response time of the frequency-stabilization loop. The
maximum amount of frequency tuning is limited by the phase
excursion of the phase shifter 85. This phase-lock loop also is
effective in stabilizing the oscillation frequencies against
environmental perturbations that can change the lengths of the
delay lines 34, 36 and propagation delays of the
oscillator-feedback loops and thereby cause the frequencies of the
RF tones generated by generator 14 to drift. Use of a phase-lock
loop for frequency stabilization of a photonic oscillator is known
per se in the art. However, such loops have not been used before to
deliberately tune the frequencies of the RF tones generated by a
photonic oscillator.
A second approach for controlling the tone spacing and the
frequencies of the tones is shown in FIG. 5. In this case, the
optical intensity modulator is an electro-absorption modulator 32',
which is known per se in the art. The electro-absorption modulator
32' also acts as a photodetector and produces an electrical version
of the RF comb in addition to an optical version of the comb which
is supplied to the delay lines. The electrical output of modulator
32' is frequency-divided by a divider 87, filtered by a filter 83
and phase-detected by a mixer 86 to produce an error signal 90 at
the output of loop filter 88. The error signal 90 is used to adjust
separately the physical lengths of the two optical-fiber delay
lines in the two loops 34 and 36 by a fiber length control 89 and
thereby tune the frequencies of the tones. Alternatively, one
detector can be used instead of two detectors 38, 40. See U.S.
provisional patent application serial No. 60/332,372 filed on Nov.
15, 2001 and its corresponding non-provisional application bearing
Ser. No. 10/116,829 filed on the same date as the present
application, both entitled "Agile Spread Waveform Generator", both
of which are mentioned above, for additional details in this regard
(the single detector appear at element 39 in FIG. 10 thereof). This
frequency-control approach also can be used to stabilize the tone
spacing and the precise frequencies of the tones against
environmental perturbations. In this case, the error signal 90 is
used to compensate for changes in the physical lengths or
refractive indices of the loops 34, 36 produced by environmental
perturbations. Note that the phase delays of the two optical
feedback loops can be controlled separately. Thus, this approach is
less dependent on the frequency pulling capability of the two
coupled oscillator loops.
A multi-tone amplitude modulator 15 is used in the agile waveform
synthesizers of FIGS. 1 and 2. Multiple variations of this
modulator 15 are described herein. These variations are based,
however, on known techniques that have been developed previously
for add/drop filtering with the frequency resolution appropriate
for dense-wavelength-division multiplexing (DWDM) applications.
Generally, a frequency resolution of 50 200 GHz is needed for DWDM
applications. In contrast, for the RF-lightwave waveform
synthesizer, a frequency resolution on the order of 0.1 10 GHz is
preferred.
Two embodiments of the multi-tone amplitude modulator 15 are
illustrated by FIGS. 6(a) and 6(b). These modulators 15 have some
similarity to the micro-ring resonators described in prior art
document (3) identified above. The present modulator 15 contains an
optical waveguide trunk 100 that is coupled optically to multiple
circular microresonators 102. Each microresonator 102 has a
slightly different diameter. One or more electrical control lines
104 are supplied to each of the microresonators 102. The control
signals on these lines 104 adjust the optical refractive index
and/or the optical absorption of the associated microresonator 102.
If laser or electro-absorption structures are used, the single
control line 104 to each structure will control both frequency and
amplitude. Optional outlet waveguide segments 106 can be optically
coupled to each of the microresonators 102. Light incident on the
optical waveguide trunk 100 is in the form of multiple RF tones
(f.sub.1, f.sub.2, f.sub.3, f.sub.4, . . . ) that are amplitude
modulated onto a single-wavelength lightwave carrier. Each tone has
a specific lightwave frequency and generally both upper and lower
amplitude-modulation sidebands would be represented in the comb.
Light exiting the optical waveguide trunk is comprised of the same
tones but the amplitudes of those tones have been adjusted by
different weights (a, b, c, d, . . . ). These weighing factors a,
b, c, d, . . . are all less than or equal to unity unless the
resonator has gain, in which case the weighing factors a, b, c, d,
. . . are less than, equal to or greater than unity.
Optical coupling between the trunk 100 and the microresonators 102
is generally by means of the evanescent fields of the optical
modes. The optical guided modes of the trunk 100 and the
microresonators 102 overlap and can exchange energy. This coupling
mechanism is well known in the art. The relevant modes of the
microresonator propagate along the perimeter of the circular
structure. For a solid microresonator 102, these modes are known as
whispering-gallery modes. Each microresonator 102 has a slightly
different diameter and thus a different resonant optical
wavelength. One microresonator 102 is matched to each of the tones
in the comb. Coupling into the microresonators 102 is enhanced at
the resonant wavelengths. Each microresonator 102 has multiple
resonate wavelengths whose spacing is the free-spectral range of
the resonator. In practice, the resonators 102 can be designed so
that the free-spectral range (FSR) is larger than the total
bandwidth of the input RF-lightwave comb. The FSR of a circular
microresonator is equal to c/.pi.n.sub.ed, where c is the speed of
light, n.sub.e is the optical refractive index of the propagating
mode (the effective index), and d is the diameter of the resonator.
For example, a diameter of 0.3 mm would result in a FSR of
approximately 100 GHz, for an effective index of 3.2. These are
representative values for a multi-tone amplitude modulator
fabricated from waveguide structures in typical electro-optic
materials such as GaAs or InP. Note that a change in the resonant
wavelength of 1 GHz can be accomplished by a micrometer-sized
change in the diameter of the resonator. Such dimensional control
is well within the capabilities of current photolithographic
processes.
The microresonator 102 is preferably fabricated from an
electro-absorptive material, such as GaAs or InP based
semiconductors (see the embodiment of FIG. 6(a)) or an
electro-optic material such as Lithium Niobate (see the embodiment
of FIG. 6(b)). The waveguide trunk 100, however, may be fabricated
from either an electro-optic material or a non-electro-optic
dielectric (such as silica). By fabricating the microresonator 102
from an electro-optic material, one can electrically modify the
effective index of the resonator mode and thus the resonant
frequency. Such modification can be employed to compensate for
fabrication inaccuracies. It also can be used to tune the resonant
frequency in cooperation with the frequency tuning of the optical
comb generator (discussed above). A maximum change in the
refractive index of 1 5% is typical for electro-optic tuning.
In the embodiment of FIG. 6(a), the optical absorption can be
adjusted by applying a bias voltage on lines 104 across a PIN diode
structure constructed from the GaAs or InP based semiconductor
material. The optical absorption can be changed to control the
amount of optical power that is coupled from the waveguide trunk
100 into the resonator 102. As the resonator absorption is
increased, the amount of light that remains in the trunk 100 and
bypasses the resonator 102 also increases. The use of absorption
changes to modify the resonator-trunk coupling is discussed in the
references for prior art document number 3 identified above. In one
example (see prior art document (3)), a change in absorption of 20
dB resulted in a change in throughput power of 10 dB. The filter
bandwidth of the resonator-trunk combination is dependent to some
extent on the optical power lost to the resonator. Since each
resonator modifies the amplitude of only a single tone, pass band
shape is not critical so long as the filter gain/phase remains
approximately the same at the frequency of the tone being modulated
by the filter.
In the embodiment of FIG. 6(b), an optical directional coupler 105
is used with each microresonator 102 to adjust the coupling and the
control voltages on lines 104 are applied to this coupler 105.
Lines 107 can be used to control the frequency at which the
microresonators function.
To learn more about fabrication techniques for multi-tone amplitude
modulators the reader is directed to the prior art documents
referred to above and to a provisional patent application
identified above, entitled "Waveguide-Bonded Optoelectronic
Devices" bearing Ser. No. 60/332,370 and filed Nov. 15, 2001,and
its corresponding non-provisional application bearing Ser. No.
10/116,800 and filed on the same date as the present
application.
Two additional embodiments of the multi-tone amplitude modulator 15
are illustrated by FIGS. 7(a) and 7(b). This modulator 15 is based
on the add/drop filters constructed from distributed-feedback
optical resonators of prior art reference number (4) identified
above. The modulator 15 includes a waveguide trunk 100 that carries
the RF-lightwave comb. The trunk 100 is coupled to multiple taps
110, with each tap 110 being a distributed-feedback optical
resonator 110. Each distributed-feedback optical resonator 110 is
resonant at a particular optical wavelength and selects a specific
tone of the comb. Each optical resonator 110 also is coupled to an
outlet waveguide segment 112. The outlet waveguides 112 in
combination with their associated optical resonators 110 provides a
controllable means for light coupled from the trunk 100 to be
leaked away. Electrical control signals on lines 114 control the
effective index of the resonator mode (and thus the resonant
frequency) while electrical signals on lines 106 control the
optical coupling coefficient between the resonator 110 and the
outlet waveguide 112. Both the resonator and the outlet waveguide
are preferably constructed from electro-optic materials (such as
GaAs or InP based semiconductors or Lithium Niobate). Use of
electro-optic materials permits the effective indices of the modes
to be modified electrically.
Narrow resonator linewidths can be achieved by using weak coupling
between the trunk 100 and the resonators 110. Also, narrow
linewidths are achieved by having strong distributed feedback,
leading to shorter resonators compared to the transfer length for
the coupling. For linewidths on the order of 1 GHz, the desired
transfer length for coupling between the trunk 100 and the
resonator is on the order of 10 mm. Note that the linewidth
achieved with a distributed-feedback optical resonator is much
smaller than the linewidth that is possible when only the
distributed reflection is employed (such as in prior art reference
number (5) identified above). This improvement is on the order of
the ratio between the coupling strength and the square of the
distributed feedback efficiency.
The disclosed multi-tone amplitude modulator 15 of FIG. 7(a)
provides a means to electrically control the amount of coupling
from a distributed-feedback optical resonator 110 to its outlet
waveguide 112. This control is achieved by applying a voltage
across the gap 111 between the optical resonator 110 and the outlet
waveguide 112. The applied voltage changes the optical refractive
index of the material in the gap and thus the shapes of the
evanescent optical fields in that gap. Alternatively, a voltage can
be applied just to the outlet waveguide 106 to change its
refractive index. Note that one can permit the wavelength linewidth
for the coupling between a resonator 110 and its outlet waveguide
112 to be larger than the linewidth for the coupling between that
resonator and the trunk 100. The latter linewidth determines the
wavelength selectivity of the tap. As the coupling strength between
a resonator and its outlet is changed (by means of the control
voltage on lines 106), the amount of energy leaked from that
resonator 110 is changed. In terms of the coupling between the
resonator 110 and the trunk 100, this leakage out of the
distributed-feedback resonator 112 acts in the same way as the
absorption of light within the circular microresonators 102 of the
embodiment of FIG. 6(a).
The amount of light coupling to the outlet waveguide segments 112
is signified by the letters .delta..sub.1, .delta..sub.2, . . . in
FIGS. 7(a) and 7(b).
The embodiment of FIG. 7(b) is rather similar to the embodiment of
FIG. 7(a) except that the resonators 110 are used for coupling to
segments 112, not for the purpose of the leaking out or trapping
out the undesired portions of the optical signals on trunk 100 (as
in the case of the embodiment of FIG. 7(a)), but rather for the
purpose of collecting the desired portions from an input trunk 100a
and communicating them to an outlet trunk 100b via resonator pairs
110a and 110b. Resonators 110a and associated with the input trunk
110a while resonators 110b are associated with the output trunk
110b. The outlet waveguides 112 in combination with their
associated optical resonators 110a provide a frequency selective
controllable means to couple light from trunk 100a to trunk
100b.
Amplitude/phase control voltages are applied on lines 104 which are
coupled to amplitude/phase control devices 108. These devices 108
can be implemented using liquid crystal spatial modulators.
Frequency control voltages are applied on lines 114 that control
resonators 110a and 110b. If the input resonators 110a are
identical to the output resonators 110b,then for an associated pair
of resonators (in this embodiment each input resonator is
associated with an output resonator with which it is coupled by
means of an amplitude/phase control device 108, there being an
associated pair of resonators 110a, 110b for each RF tone f.sub.n
in the waveguide trunk 100a) their control lines 114 may be
connected to a common source.
To summarize, this invention consists of new constructs that
perform synthesis of RF-lightwave waveforms and RF waveforms. The
synthesis approach generates a comb of tones, controls the
frequencies of those tones and controls the amplitudes of those
tones. Two key components of the construction are based on prior
art but add new features to the prior art. For example, the
prior-art optical comb generators have been adapted to have the
capability for real-time controlling and tuning of the specific
frequencies in the multi-tone RF-lightwave waveform. For another
example, the frequency selective amplitude modulators of this
invention are based on prior art on optical add/drop filters for
DWDM. This disclosure has discussed means for adapting these prior
approaches to the task of selecting the individual tones of a
RF-lightwave comb, which have much smaller frequency spacing than
the DWDM channels. This application also has disclosed means to
incorporate real-time electrical control of the gain or throughput
of the filtering elements. By modifying the gain of these
frequency-selective taps, one modifies the amplitudes of their
associated frequency tones. Such deliberate gain control was not
needed for the DWDM filters and was not a part of the prior
art.
A number of embodiments of the invention have been disclosed and it
is likely that modification will now suggest itself to those
skilled in the art. As such, the invention is not to be limited to
the disclosed embodiments except as specifically required by the
following claims.
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